Free piston gas generator



Aug E, 194%. D. E. DAVIS v ,478,,37

FREE PISTON GAS GENERATOR Filed July 9, 194

wander m ibis D 0W3 Patented Aug. 9, 1949 FREE PISTON GAS GENERATORDavid fEdward Davis, London, England, assignor to Vwkers-ArmstrongsLimited, London, England, a British company Application July 9, 1946,Serial No. 682,341 In Great Britain July 17, 1945 2 ()laims. 1

This invention relates to free piston gas generators of the type inwhich the compressor pistons deliver, during the outward stroke of thecombustion pistons in the combustion chamber, compressed air to be fedto the combustion cham her.

With existing free piston gas generators of the aforesaid type, whenoperating at reduced working pressure the compression pressure in thecombuston cylinder reaches an undesirably high value, and introduces thepossibility of the 0pposed ends of the combustion pistons in suchcylinder contacting with each other. In order to obviate thispossibility devices have been proposed heretofore which regulateautomatically the pressure in cushion cylinders provided to store on theoutward stroke no more than the required amount of energy for the returnor inward stroke, to ensure stability under all conditions of load andoperation. However, such heretofore proposed automatic control means forregulating the cushion pressure have been undesirably complicated andthe object of the present invention is to provide cushioning means whichwill operate over the required range of power and not involve acomplicated mechanism and arrangement of valves.

According to the present invention, gaseous fluid compressing means isprovided to create a gaseous fluid stabilising cushion to absorb thesurplus energy tending to bring the pistons in the combustion chamberinto contact with each other during the compression stroke, Whilstmaintaining the compression pressure in the combustion chamber withinthe range required for efficient operation of the free piston gasgenerator, and the arrangement is such that the energy acquired orstored by reason of this absorption of surplus energy is returned to themoving parts of the gas generator at the commencement of the firingstroke of the pistons in the combustion chamber.

In carrying the present invention into practice, there is introduced anauxiliary stabilising cushion which prevents the building up ofundesirab-ly high compression pressures in the combustion cylinder, andenables the free piston gas genrator to remain stable in operation overthe full range of power without resorting to complicated stabilisingmechanisms proposed heretofore. For this purpose the normal piston-likeenlargements provided on the combustion pistons or connected theretonear the ends remote from the combustion chamber are arranged so as tooperate between the normal air compressor space and an kill aircompression space in which the stabilising pressure is generated. Forexample, in a symmetrical arrangement of a free piston gas generator,the ends of the pistons remote from the combustion chamber are formedwith the said piston-like enlargements which operate in cylindersforming the air compressors in which the required volume of air iscompressed and delivered to the combustion cylinder in the normal wayand the energy on the inward stroke results from the compressorre-expansion together with the expansion of air in relatively small butconcentric extensions of the aforesaid compression chambers, theseextensions forming the main cushion cylinders receiving rams comprisingcon centric co-axial extensions of the piston-like enlargements of thepistons in the combustion chamber. These cushioning cylinders can be ofthe known type.

The stabilising pressure is generated in the annular spaces surroundingthe combustion pistons between the opposed faces of the two piston-likeenlargements of such pistons and the inner opposed ends of the cylindersin which they operate, the required quantity of air being trapped insuch annular spaces by any convenient means such as, e. g., by thecovering of ports over the appropriate period by the two pistonlikeenlargements, following which covering of the ports the air required forobtaining the stabilising pressure is compressed, resulting in thesurplus energy being absorbed and the pistons arrested on thecompression stroke in the combustion cylinder before the opposed ends ofthe two pistons can contact or the compression pressure becomeexcessive.

The stabilising cushion, as described, can also be applied to anarrangement of free-piston gas generator in which instead of utilisingthe aforesaid co-axial extensions of the air compression cylinders ascushioning cylinders, the reverse arrangement is adopted and the spacesbetween them and the opposed or outer faces of the piston-likeenlargements of the combustion pistons used as the spaces in which iscompressed the air supplied to the combustion chamber. That is to say,the annular space surrounding each concentric ram-like extension of eachpistonlike enlargement comprises the air cushioning space, and thespaces between the ends of such rams and the co-operating extension ofthe cylinders form the air compression spaces for delivery of air to thecombustion cylinder.

In order that the invention may be clearly understood and readilycarried into effect, drawings are appended hereto illustrating, somewhatdiagrammatically, alternative embodiments thereof, and wherein- Figure 1is a sectional side elevation view of one embodiment; and

Figure 2 is a sectional side elevation view of another embodiment.

Referring to the drawings, both embodiments introduce an auxiliarystabilising cushion which prevents the building up of undesirably highcompression pressures in the combustion cylinder, and enables the freepiston gas generator to remain stable in operation over the full rangeof power.

In the embodiment illustrated in Figure 1, the combustion chamber isindicated by the reference numeral and the free pistons by the referencenumerals 2 and 3, the cylinder 4 appropriate to the combustion chamberbeing connected across air compression cylinders 5 and 6 in whichreciprocate pistons l and 8 respectively formed as enlargements of thepistons 2 and 3. The required volume of air to be compressed anddelivered to the combustion chamber I is obtained by the compression ofthe air in the cylindrical enlargements 5 and 6 between the pistons Tand 8 and the outer ends of such cylindrical enlargements.

The pistons I and 8 carry concentric extensions II and I2 respectivelyoperating in cylindrical extensions [3 and M of the cylindricalenlargements 5 and 6 respectively, such extensions [3 and ['4 formingthe main cushion cylinders and providing, during the combustion state inthe cylinder 4, a cushioning action, and during the finward strokeproviding, together with the air compressor re-expansion energy, therequired reexpansion energy for driving the pistons 2 and 3 towards eachother to compress the air in the combustion chamber I to ignitiontemperature. The walls of the extensions l3 and M are formed with portsI 3a and Ma which open to atmosphere, the opening and closing of theseports being controlled by the extensions H and [2.

To prevent contact of the opposed ends of the pistons 2 and 3 during thecompression stroke when operating at reduced working pressure, auxiliarystabilising cushions are provided in the form of annular extensions 5aand 6a on the inner ends of the compression cylinders 5 and 6,

the extensions being arranged to inter-act with the inner faces of thepistons 1 and 8 to compress the air in said extensions 5a and Ea duringthe end of the inward stroke of said pistons and following the coveringof ports l5 and I6 cformed near the inner ends of the compressioncylinders 5 and 6 respectively. This results in the surplus energy,which otherwise may cause the opposed ends of the pistons 2 and 3 tocontact to be absorbed so that the pistons 2 and 3 are arrested on theircompression strokes within safe mechanical limits at the same time formaintaining the compression pressure in the combustion chamber withinthe range appropriate for the most eiiicient operation. It will beapparent that the energy stored within the annular extensions 5a and 6awill be returned to the moving parts at the commencement of the firingstroke thereby obtaining an arrangement in which increased aceelerationof the pistons takes place at the most appropriate stage giving improvedemciency of combustion.

In the modified arrangement shown in Figure 2 the stabilising pressureis still provided for by means of the annular extensions 5a and 6abeiii) tween the inner or opposed faces of the pistons 1 and 8 and theinner end walls of the cylindrical enlargements 5 and 6, but thecompression of the required volume of air for delivery to the cylinder 4is effected by operation of the concentric extensions II' and 12 of thepistons l and 8 within chambers I! and I8 occupying the same position asthe cushioning cylinders l3 and I4 illustrated in the arrangement shownin Figure 1, the required re-expansion energy for driving the pistons 2and 3 towards each other to compress the air in the combustion chamber lto ignition temperature being derived by the re-expansion of the air inthe spaces in the cylinders 5 and 6 between the outer faces of thepistons l and 8 and the outer end walls of such cylindricalenlargements, together with the air compressor re-expansion energy. Thatis to say, the arrangement illustrated by Figure 2 in effect exchangesthe positions of the chamber shown in Figure l for supplying thecompressed air to the cylinder 4 and the chamber forming the main aircushion providing re-expansion energy. By this arrangement thestabilising cushion and the main cushion are located within the samecylinder and the cylinder parts may be common to both, as i1- lustrated.

I claim:

1. A free-piston gas generator of the type specified comprising acombustion cylinder, opposed combustion pistons operating in saidcylinder, air compression cylinders mounted co-axially with saidcombustion cylinder one at each extremity thereof, piston-likeenlargements mounted one on the end of each combustion piston remotefrom said combustion cylinder and arranged to reciprocate in said aircompression cylinders to provide compressed air which is fed to thecombustion cylinder, main pneumatic cushioning means associated with theends of said air compression cylinders to provide a cushioning actionduring the outward stroke of the said pistons and piston-likeenlargements and to provide part of the expansion energy for driving thepistons on the inward stroke, annular extensions on the inner end ofeach said compression cylinder said extensions being arranged tointeract with the inner faces of said piston-like enlargements tocompress the air in said extensions during the end of the inward strokeof said pistons and piston-like enlargements to provide auxiliarystabilizing cushions said auxiliary stabilizing cushions being at othertimes freely in communication with atmosphere through ports in the wallsof said compression cylinders adjacent said extensions.

2. A free-piston gas generator of the type specified comprising acombustion cylinder, opposed combustion pistons operating in saidcylinder, cylindrical enlargements formed at each end of said combustioncylinder, piston-like enlargements mounted one on the end of eachcombustion piston remote from said combustion cylinder and arranged toreciprocate in said cylindrical enlargements to provide a stabilizingcushion during the outward stroke of said pistons and piston-likeenlargements by compression of the air in said cylindrical enlargementsbetween the outer ends thereof and the outer faces of said piston-likeenlargements, and also to provide part of the expansion energy necessaryfor driving the pistons on the inward stroke, annular extensions on theinner end of each said cylindrical enlargement, said extensions beingarranged to interact with the inner surfaces of said piston-likeenfreely open to atmosphere at other times and air 10 compression meansassociated with said cylindrical enlargements for providing compressedair which is fed to the combustion cylinder.

DAVID EDWARD DAVIS.

6 REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,757,215 Pescara May 6, 19302,076,258 Witkiewicz et a1. Apr. 6, 1937 2,090,709 Steiner Aug. 24, 19372,406,037 Ramsey Aug. 20, 1946

